Having access to the data set itself could unlock a lot of new, creative ideas and applications beyond the expected ones. That's one great thing I've learned from the open source community.
It could not only be used for search, but some data analysis, and what not. I think it would be fairly beneficial for github to do it, actually. Easy to work with, up to date dataset -> interesting projects -> github brand value++.
Or go the other way, and train a searching algorithm based on a data set of actively maintained open source projects on Github, with things like commit history/contributors/etc as features.